A study recently published in the U.S journal Proceedings of the National Academy of Sciences helped explain the popular scientific finding that women with multiple sons are more likely to have a younger gay son with older brothers. This finding also lends credence to several other scientific findings including one of many gay gene studies before it.

Why men with older brothers are more likely to be gay

The scientists believe that the more often a mother is impregnated with a male fetus, the more likely she is to develop antibodies that prevent specific DNA protein strands on Y-chromosomes associated with male brain development. These proteins, known as PCDH11Y and NLGN4Y, alter brain structures that may affect “underlying sexual orientation in their later-born sons.”

To test their hypothesis, researchers examined the blood plasma of 142 women and 12 men ages 18 to 80. The group of women included those who had sons (about half of them had gay sons) and women without sons.

Researchers found that women overall had more anti-NLGN4Y antibodies than men. They also found that mothers of gay sons, particularly those whose sons also had older brothers, had significantly higher levels of anti-NLGN4Y antibodies in their plasma than did women with no children and mothers with heterosexual sons.

The study would have to be replicated in order to test out its verifiability, but it possibly supports a 2006 study showing that a man’s chance of being gay goes up by about a third with each older brother he has (though that study didn’t explain why).

Gay gene studies are advancing our overall understanding of genetics

This study also lends credence to another recent scientific discovery showing a distinct difference in the genetic code of gay and straight men. That study also focused on a DNA strand affecting male brain development.

This finding also supports the theory in a viral 2016 TED Talk that epigenetics — structures that determine how similar genes toexpress themselves in different ways based on external circumstances — give gay sons to mothers with multiple male offspring as a way to minimize resource and mate competition among brothers and help with family’s emotional health and cohesion.